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(DAMASCUS, Md.) -- A pregnant Maryland high school teacher who has been missing for more than a week has been found dead in a shallow grave, and her boyfriend is charged with her murder, Montgomery County police announced Wednesday night.

Police Chief Thomas Manger said investigators found Laura Wallen's body in a shallow grave in Damascus, Maryland, about 38 miles west of Washington.

Wallen's boyfriend, Tyler Tessier, was arrested Wednesday afternoon and charged with murder, Manger said, adding that the motive is unknown.

The cause of death is unknown, Manger said. An autopsy will be performed.

Wallen -- who was four months pregnant -- vanished last week after she contacted relatives in a text message, authorities said.

The Montgomery County Police Department previously said in a news release that Wallen's family received the text on the morning of Sept. 4.

It's unclear what the message contained, and police did not respond to ABC News' request for comment.

Police believe it's the last reported contact with Wallen, 31, because she was absent on the first day of school, according to the news release.

"Laura, if you're listening, it doesn't matter what's happened, it doesn't matter what type of trouble, there's nothing we can't fix together, myself and your family," Tessier, said through tears during a news conference.

Montgomery County investigators had said they did not suspect foul play, but Tessier made a direct appeal to anybody who would harm his girlfriend and unborn child.

"If somebody has her, please understand you've taken away a huge, a huge person that's in so many lives," he said at the news conference, emphasizing that there wasn't anything strange in her behavior before she disappeared.

"I know what she means to me, and I know what she means to everybody else," he said. "We just want to know she's OK. We just want her back."

Laura Wallen's father, Mark Wallen, said at the news conference, "I wanted to let Laura know that we're out there looking for her and we're not giving up on her," and added that she had been "superexcited" about the school year.

He refused to believe something grim has befallen her.

Police confirmed in the release last week that they discovered her 2011 black Ford Escape parked in a gated apartment complex almost directly across a highway from Wilde Lake High School.